The present invention relates to controlling torque in an internal combustion engine to provide a smooth torque transition in response to cylinder deactivation and reactivation, transmission shifts, and increase and decrease in compression ratio or to provide a desired torque transition in response to a traction control event or driver demand.
A variable displacement engine (VDE) is one in which a portion of the cylinders of a multi-cylinder engine may be deactivated, typically for improving engine efficiency under some operating conditions. The highest thermal efficiency of an engine occurs at an engine torque that is approximately 75% of peak engine torque. Driver demand for torque, however, is often well below the peak efficiency torque level. The VDE improves efficiency by operating fewer than all cylinders closer to the peak efficiency point.
One of the problems encountered in developing a vehicle with a VDE for production is making the transitions from the situation with all cylinders active to partial cylinder activation and the reverse. For example, if four cylinders of an eight-cylinder engine were active and the operator of the vehicle demanded more torque than the four cylinders could provide, the deactivated four cylinders may be activated. The airflow to the engine nearly doubles immediately upon cylinder reactivation as now eight cylinders, instead of four cylinders, are drawing air from an intake manifold, which is at high pressure, and a torque disturbance ensues.
To rapidly change torque to allow a smooth transition for VDEs, the throttle may be closed rapidly to restrict the airflow at the same time that the cylinders are reactivated. The effect of closing the throttle occurs over a number of engine events, i.e., not instantaneously. The inventors herein have recognized than an instantaneous change is necessary to smooth the torque fluctuation during a VDE transition or other types of transitions in internal combustion engines, which are accompanied with a torque fluctuation.
In U.S. Pat. Nos. 5,437,253 and 5,374,224, assigned to the assignee of the present invention, and U.S. Pat. No. 5,481,461 spark retard is used to accomplish a smooth transition, where a transition may be a deactivation or reactivation of cylinders. As spark timing is retarded from MBT (minimum spark advance for best torque), torque is reduced. Control of spark timing is a desirable tool to use for immediately affecting torque as a change can be made effective in the next engine combustion event. Using spark timing alone, however, may not provide enough torque diminution to provide a smooth torque trajectory during the transition. Furthermore, depending on the range in spark advance allowed by the engine controller, there may be operating conditions at which sufficient spark retard is not accessible. The inventors herein have recognized that an alternative or additional measure to reduce torque in the event of a transition is needed.
EP0937880 discloses a method by which air-fuel ratio is varied to control torque to the desired level during a transition. The inventors of the present invention have recognized that air-fuel ratio excursions away from a stoichiometric proportion, occurring within an aftertreatement device, is an unsuitable approach in an engine equipped with a three-way catalyst in which the catalyst function depends on the air-fuel ratio being maintained at stoichiometry.
In U.S. Pat. No. 4,006,722, air-fuel ratio is varied among cylinders for the purpose of reducing NOx produced by the engine. All the cylinders are supplied with a rich air-fuel ratio mixture. A subset of the cylinders is supplied with supplemental air such that the subset is at a lean air-fuel ratio. The inventors of the present invention have recognized that with electronic port fuel injection, lean and rich air-fuel ratios can be supplied to cylinders without the need for additional hardware to provide supplemental air to the cylinders. The inventors have further recognized that electronic port fuel injection allows supplying a rich or lean mixture to as few as one cylinder; whereas, in U.S. Pat. No. 4,006,722, which relies on a central carburetor, a rich mixture is supplied to all cylinders.
In U.S. Pat. No. 4,006,722, additional fuel is supplied to all of the cylinders and additional air is supplied to a subset of cylinders. Both measures lead to a torque increase. The inventors of the present invention have determined an alternate method for supplying a rich mixture to some cylinders and a lean mixture to other cylinders which causes a torque decrease.
A mode transition method is provided for controlling torque produced by an internal combustion engine. The engine has a plurality of cylinders, an exhaust system containing one or more emission aftertreatment devices, and an engine controller operably connected to the engine for controlling the relative air-fuel ratio supplied to the cylinders. The method includes the steps of operating at least one cylinder at a lean relative air-fuel ratio; and operating at least one other cylinder at a rich relative air-fuel ratio to reduce emissions which would otherwise be caused by operating at least one cylinder at a lean relative air-fuel ratio. The cylinder at a lean relative air-fuel ratio and the cylinder at a rich relative air-fuel ratio provide a desired relative air-fuel ratio to the aftertreatment device, which may be a stroichiometric air-fuel ratio. A desired torque during the mode transition may be computed. The rich relative air-fuel ratio in the rich cylinders, the lean relative air-fuel ratio in the lean cylinders, and a retarded spark timing provide the desired torque.
A system for controlling torque during a transition of operating mode in an internal combustion engine is disclosed. The engine has a plurality of cylinders, an engine exhaust system containing one or more emission aftertreatment devices, a throttle valve disposed in an air intake duct, and an engine controller coupled to the engine for controlling the relative air-fuel ratio supplied to the cylinders. The engine controller provides to at least one cylinder a lean relative air-fuel ratio and to at least one other cylinder a rich relative air-fuel ratio to reduce emissions which would otherwise be caused by operating a cylinder at a lean relative air-fuel ratio. The engine controller also computes a desired throttle valve position based on a desired torque during the transition in operating mode.
Prior art methods for reducing engine torque include providing a lean relative air-fuel ratio to some engine cylinders. The present invention overcomes problems of prior art methods by providing a lean relative air-fuel ratio to some cylinders and a rich relative air-fuel ratio to some other cylinders. Prior art methods lead to a lean relative air-fuel ratio being delivered to the exhaust aftertreatment devices, which causes the aftertreatment device efficiency to degrade markedly if it is a three-way catalyst. The advantage of the present invention is that a desired relative air-fuel ratio is delivered to the exhaust aftertreatment devices, which may be a stoichiometric relative air-fuel ratio.
By using unequal fueling to cylinders, the present invention improves on the prior art method of retarding spark timing. The advantage is that unequal fueling may be combined with spark timing to provide a greater range of authority in controlling torque than retardation of spark timing alone.
Other advantages, as well as objects and features of the present invention, will become apparent to the reader of this specification.